Fix egg_info.writers entry point module path (#829)

[danieldk]

Backport of #829 to 0.17.

Problem

With kernels installed, any setuptools source build in the same environment fails, including builds of unrelated packages:

File "setuptools/command/egg_info.py", line 304, in run
    writer = ep.load()
ModuleNotFoundError: No module named 'kernels.lockfile'

Reproducer:

pip install kernels==0.17.0
printf 'from setuptools import setup\nsetup(name="smoke", version="0.1")\n' > setup.py
python setup.py egg_info

Cause

kernels/pyproject.toml registers a setuptools entry point pointing at a module that is not shipped:

[project.entry-points."egg_info.writers"]
"kernels.lock" = "kernels.lockfile:write_egg_lockfile"

write_egg_lockfile lives in kernels/locking.py; there is no kernels/lockfile.py in the wheel. The entry point was not updated when lockfile.py was renamed to locking.py.

setuptools loads all registered egg_info.writers on every egg_info run, so a broken writer from any installed distribution takes down unrelated builds. PEP 517 isolated builds are unaffected; --no-build-isolation builds reuse the ambient setuptools and always hit it.

Present in 0.17.0 and on main; 0.16.2 is the last good release.

Fix

Point the entry point at kernels.locking, and add a regression test that loads it the way setuptools does — nothing covered this path, which is why the rename regressed silently.

Validation

Built a wheel from this branch and installed it:

• ep.load() resolves to the real function.
• The reproducer above succeeds.
• The new test passes, and fails when reverted to kernels.lockfile.

[HuggingFaceDocBuilderDev]

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